An electric machine (motor or generator) is an apparatus converting energy between electric power and mechanical rotary motion. There are different types of electric machines including induction machines, permanent magnets machines, switching reluctance machines, synchronous reluctance machines and hybrid machines. The various embodiments in this disclosure are applicable to the different types of electric machines above, which are configured as either motors or generators. Induction motors as an example are used to illustrate the innovative aspects of the present disclosure. The induction motor comprises a stator and a rotor. The stator is the stationary part and the rotor is the rotating part. The rotor may be inside the stator, outside the stator or beside the stator as in an axial field machine. An induction motor having a rotor inside a stator is used as an example to illustrate the innovative aspects of the present disclosure. A small motor air gap exists between the rotor and the stator for mechanical clearance and mechanical torque generation.
The squirrel cage inductor motor is the most common electric machine. The stator of the squirrel cage inductor motor comprises a plurality of windings. The plurality of windings usually forms a plurality of phase belt arranged in pole pairs. The rotor of the squirrel cage induction motor comprises a shaft and a squirrel cage made of metal bars contained in a magnetic structure such as a laminated silicon steel stack. The shaft is surrounded by the metal bars. First ends of the metal bars are connected by a first interconnect ring. Second ends of the metal bars are connected by a second interconnect ring.
In operation, the electric power is usually applied to the stator. As a result, a first magnetic field is created in the stator and in the air gap. The first magnetic field rotates in time at a synchronous speed with alternating current (ac) power applied to the stator windings. The first magnetic field induces electric currents in the metal bars of the rotor. The induced current produces a second magnetic field in the rotor. The second magnetic field of the rotor reacts against the first magnetic field of the stator. According to Lenz's Law, the rotor follows the rotating first magnetic field and generates a mechanical torque pulling the rotor into rotation. In a motor mode, the rotor will fall behind the first magnetic field. The speed difference between the first magnetic field and the rotor keeps inducing the electric current inside the rotor. If a load is applied to the rotor and the rotor falls further behind the first magnetic field, more torque will be developed due to the lag between the rotor and the first magnetic field. In other words, the torque of the motor is approximately proportional to the slip between the speed of the rotor and the speed of the first magnetic field.
The theoretical speed of the rotor in an induction motor traditionally depends on the frequency of the electric power supply and the arrangement of poles in the stator coils. With no load on the motor, the speed of the rotor is equal to or approximately equal to the synchronous speed of the rotating magnetic field. The synchronous speed of an inductor motor is determined by the frequency of the electric power supply and the number of poles of the inductor motor. In particular, the speed of the induction motor is equal to the frequency of the electric power supply times 60 and further divided by the number of pole pairs.
As energy efficiency becomes an increasingly important issue, more motors and generators are coupled to power electronics equipment in variable speed applications, such as industrial drives, electrical vehicles, diesel-generator sets, servo systems, and wind power generation. Many of these applications require the motors and generators to operate over a wide speed and power range, and traditional technologies cannot satisfy the performance and cost requirements for such applications. Especially, as renewable energy becomes an important issue, more motors are used to drive electrical vehicles. There may be a need for having a motor operating efficiently over a wide speed and power range. Traditional motors cannot satisfy the performance and cost requirements for such applications. It has been proposed to dynamically adjust the number of poles and/or the number of phases of the motor to provide more freedom to optimize the performance of the motor, especially to improve the efficiency of the motor and the drive system. The number of poles in a machine, and/or the number of phases in a pair of poles may be changed by controlling the currents in the windings, particularly by changing the phase relationship between adjacent windings. However, how to control the motor and implement such a dynamic adjustment of number of poles and/or the number of phases has remained a significant challenge.
It would be desirable to have a high performance motor system with advanced control techniques exhibiting good behaviors such as high efficiency over a variety of speed and power range conditions at a low cost.